Angalifu, one of the world's last northern white rhinos, died at the San Diego Zoo's Safari Park on Sunday.
The rhino, 44, apparently died of old age. The Los Angeles Times reports that he had refused to eat for several days before his death.
The park attempted to breed Angalifu with the park's other northern white rhino, but the efforts were unsuccessful. The San Diego Zoo Institute for Conservation Research has saved some of Angalifu's semen in case it can one day be used with the remaining northern white rhinos.
The other four remaining animals are at a zoo in the Czech Republic and a preserve in Kenya. Poaching has driven the species to nearly complete extinction.